2017-12-13
Do plastic industry, have to know the internal stress of plastic

What is stress
Before explaining what is the internal stress of a plastic, we must first understand what is the stress when an object is subjected to external stress
The object itself will resist the external force of oppression, resistance to deformation, this resistance force, we call the internal force. Let's take a simple example, such as you use your feet to crush someone's mobile phone, at this time, of course, the mobile phone can not be indifferent, even if you can't see, but the mobile phone has reacted, the internal force between the internal parts of the interaction, in order to resist the deformation caused by your rolling.
Two, what is the internal stress of plastic
In the first point, you should know what stress is, when you move your foot away from crushing someone's phone, the part of the stress created by the phone that doesn't disappear, the part of the force that remains on the phone, is called internal stress. Plastic internal stress is a kind of internal stress, but also under the action of no external force, the stress in the internal existence. Official EXPLANATION, PLASTIC INTERNAL STRESS IS TO SHOW PLASTIC IN PROCESSING PROCESS, ESPECIALLY IN INJECTION MOLDING PROCESS, LOOK ON MICROCOSMIC, THE ORIENTATION OF CHAIN OF MACROMOLECULE INSIDE PLASTIC AND THE FACTOR SUCH AS COOLING CONTRACTION AND A KIND OF INTERNAL STRESS THAT PRODUCE.
Almost all plastics have internal stresses, just to varying degrees. Plastics such as PC and PPE are easily affected by internal stresses. Today why should we focus on explaining the internal stress, because the internal stress treatment is not good, may make the product in the use or storage process, stress cracking and warping deformation, in addition, but also affect the optical properties of products, mechanical properties and appearance. We can't eliminate the internal stress, but we can greatly reduce the internal stress as long as we find the root cause, the right medicine.
Three, the causes of plastic internal stress
1. Cooling internal stress: in the process of plastic processing, plastic molding needs to be cooled and finalized, which will produce shrinkage. If the shrinkage is uneven, internal stress will be generated.
2. Orientated internal stress: in the process of plastic processing, it enters the cavity through the gate, which is called mold filling. In the process of mold filling, pressure should be maintained.
3. Molecular chain rigidity: Molecular chain rigidity, the greater the plastic polymer molecules, the higher the viscosity of the liquid, the worse, activity, molecular chain in under the action of external force, elastic deformation occurred after the recovery performance is poor, some high polymer molecular chain containing benzene ring, such as PC, PPO, PPS and its corresponding products internal stress is larger, it is genetically determined, also like a person's genes, It can't be changed.
4 molecular chain polarity: molecular chain polarity, for this term, many people may not understand, let's put it in another way, the greater the mutual attraction force between the molecules in the plastic, the greater the mutual movement between the molecules, when subjected to elastic deformation, the recovery ability is weak.
Four, what operations in injection molding make plastic internal stress
1) Overfilling
2) The cooling is too fast, especially for crystalline plastics, which makes the internal stress too late to release or difficult to release
3) Direct sprue meat thin and shallow mouth is easy to residual stress.
4) The barrel temperature is too high